This invention relates to a turboramjet engine having a variable air intake, a ram air duct annularly enveloping the basic turbojet engine, and shutoff means to energize the ram air duct with air when the basic turbojet engine is shut off and the basic turbojet engine when the ram air duct is shut off.
Combined turbojet-ramjet (turboramjet) engines have lately attracted renewed interest in connection with hypersonic flight concepts for an extremely wide mission spectrum reaching from takeoff to high supersonic speed at high altitudes (to about 30 km). The hypersonic concept also includes a space vehicle concept (Saenger project) involving a two-stage concept as described below. The first stage is to be implemented by a vehicle operating solely within the atmosphere, while the second stage is based on a payload vehicle riding piggyback on said first vehicle and continuing its assigned trajectory in the upper region of the atmosphere by means of a suitable rocket propulsion system to perform space missions. The vehicle for the first stage is therefore recoverable and reusable, and it makes takeoffs and landings like a conventional aircraft.
On a turboramjet engine to be used for, e.g., such a vehicle, the basic turbojet engine is generally cut out continuously when a flight speed of Mach 3 is reached and the respective ramjet engine is cut in continuously to solely achieve the desired high supersonic or hypersonic speeds of up to Mach 4.5 or even higher. Airspeeds of about Mach 2 or even higher can here be achieved in combined "turbojet plus afterburner selected" operation; where the afterburner is advantageously arranged immediately downstream of the basic turbine engine and normally energized with compressor or fan air and engine exhaust gas in combination, and where with the basic turbojet engine cut out the afterburner forms the ramjet engine propulsion system, where additional fuel injection means and flame stabilizers are cut in and suitably metered ambient air is supplied as the only air source. With a suitably adapted intake and exhaust nozzle geometry such an engine thus cuts out or deactivates the basic turbojet engine in hypersonic operation, where the outer ram air duct is then energized with the requisite amount of ram air to be admitted into the additional combustion chamber for ramjet operation at a point, e.g., downstream of the basic turbojet engine end.
In this arrangement the necessary shutoff devices of the basic turbojet engine for the ramjet mode and of the ram air duct for the basic turbojet engine mode are gaining vital importance. In the shutoff or locked position of the basic turbojet engine during ramjet operation the basic engine should be compromised as little as possible by the comparatively high temperatures of the ram air (about 1700.degree. C. and above); which applies to the shutoff devices proper, but importantly also to the actuating and adjustment systems needed to operate the shutoff devices. Also the actuating and adjustment systems should preferably not aerodynamically disturb the ambient air ingested by the basic engine compressor.
It must also be ensured that as the case may be, as perhaps in subsonic operation or in hypersonic operation, optimum and aerodynamically clean admission of air from the engine intake into the basic turbojet engine, or alternatively ramjet air admission into the ram air duct, is achieved in the respective proportionate amounts needed.
Apart from the problems outlined above, special importance is accorded the control of the proper transition phases from the basic turbojet engine operation to the ramjet operation, or vice versa, where the changeover phases should not be jerky, but rather continuously consistent to permit aero-thermodynamically effective and economical operation of the overall engine over all operating conditions. Accordingly it becomes necessary, e.g. during a changeover phase from basic turbojet engine operation to ramjet operation to have a sufficiently large amount of intake air still available for permitting the basic turbojet engine to coast down, while on the other hand a comparatively large amount of ram air would have to be delivered into the ram air duct already at the beginning of the ramjet mode. In the reverse case of ramjet operation on a basic turbojet engine base--conceivably with afterburning, an abrupt flameout of the ramjet combustion chamber must initially be prevented, although sufficient intake air supply must nevertheless be available already when the basic engine is cut in.
For closing or opening the air supply to the basic turbojet engine to support the one case or the other, the use of teardrop centerbodies axially adjustable along the engine centerline has previously been proposed in combination with, e.g., axially adjustable sleeve valves serving to open or close as required the supply of ram air to the ram air duct in response to the adjustment action of the centerbody. This concept involves comparative great overall length plus considerable constructional expansion of the entire engine plant circumferentially to ensure the requisite air admission areas and paths into the ram air duct, and especially to the compressor of the basic engine--which would here be downstream of the major diameter of the centerbody with a view mainly to the necessary flow velocities and Mach numbers--are available. Such constructional circumferential expansion disadvantageously involves aggravated frontal area drag and component weight, where the latter is attributable also to the design of the centerbody proper. A centerbody of this design, whether it be axially adjustable or stationary, invariably causes disturbance and aerodynamic losses, especially in the position in which the intake air supply is admitted into the compressor of the basic turbojet engine.
An object of the present invention is to satisfy the aforementioned requirements for a turboramjet engine are by means of minimized constructional complexity, compact dimensions and maximum simplicity of the shutoff, adjustment and actuating devices.
This object is achieved by the present invention by providing an engine of the generic description wherein an air tract which communicates with the air intake and is arranged on a tubular duct carried axially towards the basic turbojet engine is expanded to an annular inlet section of the ram air duct, wherein the shutoff means are associated with quadrilateral sections of the tubular duct which are quadrilateral at least on the inlet side, and wherein the shutoff means of the inlet section are represented by an axially adjustable, internally swept sleeve valve and at least one flap which can be pivoted against a partial inlet of the inlet section, said flap closing off the sleeve valve lid-fashion in a jointly exposed position of the inlet section.
In the arrangements according to the invention, a turboramjet engine is provided which has comparatively compact changeover means to switch between turbojet and ramjet modes of operation. This advantageously eliminates all aerodynamic disturbances in turbojet operation. In accordance with preferred embodiments of the present invention at least one flap is pivoted, in the turbojet operating phase, against a wall of the upstream quadrilateral duct section (inlet flow duct section) of the air tract in an essentially horizontal and aerodynamically flush arrangement, where it shuts off a locally associated partial inlet flow area leading into the annular inlet section; while the remaining inlet areas are shut off by the annular, quadrilateral, or square or rectangular sleeve valve, which with a cross wall is located below the flap. Links, if used to actuate the flap when the sleeve valve is adjusted axially, are arranged, e.g., laterally inside the sleeve valve to prevent them from disturbing the intake airflow to the compressor of the basic turbojet engine.
When the basic turbojet engine is shut off to permit ramjet operation, the sleeve valve is deployed into the upstream end of the inner shield wall of the air tract, when the sleeve valve and with it the basic engine, are sealed off lid-fashion. In connection with the expansion of the quadrilateral inlet duct section relative to the inner shield wall, where in accordance with the present invention the expansion takes the shape of a slipover sleeve and both components together form the annular inlet duct to the ram air duct, a relatively large needed inlet flow area to the inlet section is exposed for free flow all around, particularly so in combination with a shutoff position of the flap extending transversely and obliquely to the extended engine centerline and accordingly obliquely adapted open position of partial flow areas on mutually oppositely arranged sides of the duct. In the shutoff position the links, or similar flap actuating means are arranged behind the flap, where they are well protected from the hot ram air stream. The exemplary single flap-like shutoff or sealing member, provides a smooth surface area and a snug faying fit for the shutoff position. It also readily permits of cooling without appreciable complexity of construction, and is conceivably used in conjunction with a thermally well-insulating ceramic and/or carbon fiber-reinforced material for the flap. For adjusting the flap the present invention provides an actuating device arranged centrally on the engine. Alternatively, two actuating devices are arranged oppositely from each other on the duct wall and preferably act on the actuating side of the sleeve valve. The actuating devices are advantageously arranged, especially when used multiply, for thermal protection on the inner shield wall in the inlet section or in the ram air duct, in which they would be partitioned off such that the actuating means will not obstruct the intake air flow during basic turbojet engine operation.
In combination with links arranged between the sleeve valve and at least one flap as intended in a further aspect of the present invention the overall inlet flow area to the annular inlet section of the ram air duct can, e.g. when the changeover is made from turbojet to ramjet operation, invariably be opened faster than the basic engine is sealed off by the flap at the sleeve valve. This is attributable also to the relatively short axial actuating movement the valve needs to expose the necessary inlet flow area to the inlet section of the ram air duct. When in other words ramjet operation is selected, e.g., an additional, sufficiently large inlet flow area to the inlet section of the ram air duct can be provided while the inlet flow area to the basic engine is still sufficient to permit continuously consistent coastdown of the basic engine. By suitably changing the length of links and their hinge position, several different operationally desirable variations of the respective required inlet flow area ratios (basic engine/ramjet operation) can be achieved during a changeover.
The above-mentioned advantageous criteria and features of the present invention apply similarly also if a two-flap concept is selected to implement them, where the respective annular as well as square or rectangular, or annularly box-shaped sleeve valve is sealed off by two folded flaps which in symmetrically pointed roof-shape configuration face the ram air flow when the basic turbojet engine is shut off.
Substantial advantages over the single-flap concept are found for example, in reduced overall length of the entire changeover system and in the fact that it permits--from the flow area aspect--symmetrical and aerodynamically especially favorable splitting of the ram air flowing into the annular inlet section. Also, the extremely short overall length of the changeover system associated with the two-flap concept does not involve constructional restraints on the compressor inlet geometry of the basic turbojet engine.
In the interest of low changeover forces, the present invention also assumes pressures to be balanced during the changeover phase in the ram air duct on the one hand and in the upstream section of the air tract plus the adjoining intake section on the other. The approximate pressure equilibrium can also prevail during ramjet operation in accordance with the present invention, but in the shutoff turbojet engine a moderate amount of positive pressure should advantageously be built to prevent the ingress of hot inlet flow air into the turbojet engine.
During straight basic turbojet operation a respective horizontally pivotable flap is subjected on both sides to approximately balanced pressures because the pressure prevailing in the annular inlet section acts in the ram air duct outside on axially short flap areas only, while the entire inner surface of the flap is exposed to th comparatively lower pressure from the intake air flow.
The pressure in the ram air duct is that prevailing downstream of the turbine of the basic turbojet engine at the hot gas exit with the hot gas valve of the basic turbojet engine opened in the connection path with the downstream end of the ram air duct. The invention assumes that also the downstream hot gas or nozzle-like exit end of the basic turbojet engine must alternatively be opened or shut off with a view to the turbojet/ramjet modes.
The sleeve valve conceived by the present invention represents a stable annular component strengthwise, which can be moved axially, along the direction of flow, with little if any appreciable effort; the external pressures acting on it in the shutoff position being absorbed by the valve proper.
In accordance with the present invention the entire air tract plus shutoff means, including the at least one flap plus sleeve valve, can be arranged as a relatively light-weight and short interchangeable module between an inlet flow duct arriving from the overall engine intake and respective walls or shrouds on the inlet side (basic engine, ram air duct).
The engine changeover and shutoff mechanism thus is characterized by comparatively little weight, extremely short sleeve valve travels, favorable support and guidance of the moving parts, low actuating force requirements at all changeover phases and favorable mutual synchronization of sleeve valve/flap(s) movements in keeping with said requirements. With but a single actuating system the changeover phases from one operating mode to the other can be initiated without risk using the respective momentarily needed duct or inlet flow areas for ram air (ramjet operation) or ingested ambient air (turbojet operation), with no special control provisions being required.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.